How to properly set up a transistor for saturation

Discussion in 'General Electronics Chat' started by ghulen, Jul 24, 2018.

  1. ghulen

    Thread Starter New Member

    Jan 4, 2018
    12
    1
    Why when I press the momentary switch does the collector LED stay on? I'm using a 330 resistor for the collector, 1k for the base, and feeding 5v into the circuit. (Iv tried flipping the transistor to see if I had it backwards). The goal is to press the switch and only have the emitter LED on 20180724_222014.jpg 20180724_222003.jpg
     
  2. crutschow

    Expert

    Mar 14, 2008
    20,075
    5,653
    I have no idea, but if you post a schematic of your circuit I might be able to help. :rolleyes:
     
  3. ghulen

    Thread Starter New Member

    Jan 4, 2018
    12
    1
    I'm completely new to electronics, but I tried to make a schematic for you. I hope its right 15324958548902484331519072960178.jpg
     
  4. crutschow

    Expert

    Mar 14, 2008
    20,075
    5,653
    The problem is that, when the transistor is turned on (saturated) the collector and emitter voltages are nearly the same, so both LEDs have the same voltage and both light.
    To get only the emitter LED to light when the transistor is on, add a standard diode (1N4148 or similar) in series with the collector LED.
    You may have to reduce the value of the collector resistor to get the desired LED current.

    Below is an alternate circuit that performs the same function.
    D1 is on when the transistor is on, and D3 is on when the transistor is off.

    upload_2018-7-24_23-28-13.png
     
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  5. ghulen

    Thread Starter New Member

    Jan 4, 2018
    12
    1
    1). I made an error on my schematic by drawing 2 voltage sources. Their is only 1 power source
    2). That diode made a significant difference. The collector led has a very faint light now. I suppose I'll have to mess with the resistances to get it completely off? With that being said is their a rule of thumb when trying to get a transistor in a saturated state? What should the voltage difference be for e<b>c?
    3). Thank you for your help
     
  6. crutschow

    Expert

    Mar 14, 2008
    20,075
    5,653
    Yes.
    It's not the voltage that is used to determine saturation, it's the base current.
    Typically the rule-of-thumb is to have the base current be at least 10% of the collector current (forced Beta of 10) for good saturation.
     
  7. danadak

    Well-Known Member

    Mar 10, 2018
    1,758
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    This circuit will eliminate a couple of Rs, and produce only pA in the off LED.

    upload_2018-7-25_5-47-24.png


    Suffers from brightness dropping when both LEDs on. That could be fixed
    by leaving one on all the time with fixed current, and switching the other on
    with same valued current limit R. Both LEDs on have identical current/brightness.

    2N7000 very useful in your toolboox, basically a usefull 100mA range type of
    part. Cheapo as well.

    If your switch is left open should add a R to ground at MOSFET gate, say 10K,
    to make sure it is not floating, eg. turn on due to leakage, induced noise.

    In this example switch pressed, only drain LED on, D2, switch off both on.

    Regards, Dana.
     
    Last edited: Jul 25, 2018
  8. Bordodynov

    Well-Known Member

    May 20, 2015
    1,928
    584
    See
    2018-07-25_13-57-06.png
     
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